Fix reference implementation's Web IDL files

This doesn't impact the generated code, because ReadableStreamDefaultReadResults are only ever used wrapped in Promises, which `webidl2js` currently doesn't unwrap.

Author: ninevra

reference-implementation/lib/ReadableStreamBYOBReadResult.webidl

@@ -1,4 +1,4 @@
 dictionary ReadableStreamBYOBReadResult {
-  ArrayBufferView chunk;
+  ArrayBufferView value;
   boolean done;
 };

reference-implementation/lib/ReadableStreamDefaultReadResult.webidl

@@ -1,4 +1,4 @@
 dictionary ReadableStreamDefaultReadResult {
-  any chunk;
+  any value;
   boolean done;
 };